Methods of producing ceramic molds for production of metal castings are well known. The earliest method known to applicant is that disclosed in Shaw, U.S. Pat. No. 2,795,022, and commonly referred to as the "Shaw process." In the Shaw process, the mold is fabricated entirely from a single composition consisting of a comminuted highly refractory material, a binder, water, and a gelling accelerator. The binder typically comprises a lower alkyl silicate, such as ethyl silicate. These ingredients are mixed to form a homogenous paste, and then poured about a pattern and permitted to gel. Immediately after gelling, the mold is removed from the pattern and then fired, i.e. ignited in an open-air furnace to remove the alcohol or any other burnable volatiles formed by the hydrolysis of the binder and thereby "fix" the mold dimensions. According to the '022 patent, when combustion of the volatiles ceases, the mold can either be at once used for casting metal or it can be further heated in a suitable furnace.
A similar method is disclosed in U.S. Pat. No. 2,811,760 to C. Shaw. That method is directed to the use of molding mixtures containing binders that, unlike the binder disclosed in the '022 patent, do not yield combustible volatiles (such as alcohol) upon setting. Such molds are also "fixed" by firing because the heat of the fire causes the evaporable substances in the mold to quickly evaporate and escape through the external mold surfaces.
While the Shaw process is purportedly suitable for some purposes, it has been found that molds fabricated of a single layer of highly refractory material are unsuitable for most purposes, especially where low tolerances are desired. This is believed to be for the following reasons. After the mold composition gels, it is fired. The purpose of firing the mold is to remove at least a substantial amount of the volatiles or other evaporables in the mold composition to "stiffen" or "fix" the mold so that it subsequently can be baked or, alternatively, immediately used for casting metal. The purpose of the baking step is to remove any residual volatiles or evaporables remaining in the mold. This permanently fixes the mold dimensions so they do not change during the casting stage, wherein the mold is subjected to the extremely intense heat of the molten metal.
The amount of volatiles or evaporables that are removed from the mold during the firing stage is, to some degree, related to the porosity of the mold composition which, in turn, is related to the particle size of the refractory used. If the particle size of the refractory material is too small and the mold composition is thus too dense, only the volatiles or evaporables at or near the surface of the mold will be released or combusted during the firing stage. The mold, being comprised of a highly refractory material, is an excellent insulator onto itself. Thus, the interior region of the mold will remain relatively cool and the volatiles and evaporables therein will neither escape nor combust. Thus, when the mold subsequently is baked or, alternatively, immediately used for casting metal, the volatiles or evaporables remaining in the interior region of the mold will undergo rapid combustion or gas phase transformation due to the intense heat of the furnace or molten metal. Because of the density of the mold, however, the resultant gases will be unable to either escape to the atmosphere, or combust, as quickly as they are generated. As a result, the mold will twist or distort and, in some instances, explode. Naturally, the thicker the mold, the more untoward the effects.
If, on the other hand, the particle size of the refractory material is too large and the mold composition is thus too porous, two problems occur. First, the mold will unlikely be able to withstand the heat of the molten metal. Second, even if that problem is overcome, smooth surface finishes of the metal casting are difficult to achieve. The porous, and thus rough, surface of the mold will result in a correspondingly rough surface of the metal casting. This may also affect the dimensional tolerance of the metal casting and render it unacceptable for uses requiring low tolerances. In addition, such a casting will need further machining and finishing procedures, which are labor-intensive and expensive.
Moreover, determining and obtaining the optimum particle size refractory material for a given mold is a difficult task. First, the shape, size, and configuration of a mold varies with the metal casting to be manufactured. Each customer's needs are different. Second, even if the optimum particle size could be determined for a particular mold size and configuration, commercially obtainable refractory materials have varying average particle sizes even among same grades. Thus, it is difficult to duplicate a mold exactly.
The problems of twisting and warping encountered in the Shaw process have been expressly recognized by Shaw himself in a later patent, U.S. Pat. No. 3,022,555. In that patent, Shaw disclosed a method which was proclaimed to be a solution to the twisting and warping of the molds of the original '022 Shaw process. In the '555 method, fragments or crushings of a finished, highly refractory mold made in accordance with the original Shaw process are incorporated into the slurry from which the new mold is made. In the alternative, a slab or undersized pattern can be made according to the original Shaw process, and then the highly refractory slurry is poured about the slab to create the final mold. The fragments or slabs constitute about 10-50% of the mass of the mold and about 10-50% of the volume of the mold. In either case, after the slurry gels or hardens, the mold is fired to remove the volatiles.
It has been determined, however, that the '555 method does not work well. In short, such a mold is weak and unstable and the heat of the molten metal causes the mold to break apart. In practice, the slurry does not adhere well to the crushings or slabs as its gels. In addition, whereas the gelled slurry expands when it is subsequently fired or baked, the crushings or slabs do not expand. As a result, separations occur between the surfaces of the hardened slurry and the surfaces of the crushings or slabs.
Other methods intended to improve, replace, or economize the Shaw process have also been devised. For instance, in the above-mentioned Shaw '760 patent, a method of constructing a mold of two ceramic layers--an "inexpensive" backing layer and a highly refractory facing layer--is disclosed. In this alternate method of the '760 patent, there is first produced a highly refractory facing layer by pouring an ethyl silicate base slurry over the face of the pattern and permitting it to gel. Immediately thereafter, a slurry containing an "inexpensive" refractory is poured against the facing layer to create a supportive backing layer. Immediately after the backing layer gels, the entire two-layer mold is placed in a pre-heated furnace to rapidly remove the evaporable substances.
In U.S. Pat. No. 2,931,081 to Dunlop, the backing layer is poured first and it is treated with carbon dioxide gas for hardening. The hardened backing layer is then placed over the actual pattern, and the space between the backing layer and the pattern is filled with the highly refractory mixture to form the facing layer. After the facing layer gels, the composite mold is fired to burn off the alcohol.
In both the method of the Shaw '760 patent and the method of the Dunlop '081 patent, the highly refractory facing layer and the "inexpensive" backing layer of the mold are formed and hardened, or gelled, before any firing or baking of either layer occurs.
The apparent benefit of these two-layered molds is having a very porous backing layer through which volatiles and evaporables can readily escape during both the firing and baking stages and, at the same time, having a very fine facing layer which resists the heat of the molten metal and provides a smooth casting surface. Because the facing layer is thin, the gas-capture problems associated with thick, single layer molds are avoided.
However, these two-layered molds still have not proven satisfactory for either production of metal castings requiring very low tolerances or applications where those low tolerances need to be consistent among different molds made from the same pattern. For instance, in the two-layer mold method such as that disclosed in the Shaw '760 patent, it has been learned that the best tolerance that can be achieved for a mold having a dimension up to 5 inches is .+-.5 thousandths inch. For each inch of mold cavity dimension above 5 inches, and additional 1 thousandths inch tolerance is added. For example, a mold having a cavity depth of 8 inches would have an achievable tolerance of .+-.8 thousandths inch and a mold having a cavity depth of 20 inches would have an achievable tolerance of .+-.20 thousandths inch. For many modern applications, even the best achievable .+-.5 thousandths inch tolerance is unacceptable.
Moreover, achieving the same tolerance among two molds made from the same pattern is extremely difficult and, as a practical matter, can only be attained randomly. Because the distortion that occurs in the mold during the baking or casting stages varies with the density, or porosity, of the mold composition, as well as other factors, each mold will have slightly different final dimensions. In practice, all of the parameters involved in manufacturing a mold simply cannot be duplicated exactly from mold to mold. For instance, for all practical purposes, it is very difficult to obtain commercially two volumes of refractory material having identical particle size distributions, even if the average particle size is the same. It is also very difficult to obtain, as between two molds, the exact amount and strength of each ingredient (accelerator and gelling agent), or the same gelling times and firing and baking temperatures and times.
It is now believed that these problems inherent in the prior art two-layer mold fabrication methods are due to the following reasons. When the composite two-layered mold is baked, the porous backing layer expands. This expansion of the material is both desirable and undesirable. Expansion of the backing material is desirable because the mold must be sufficiently porous to permit gases released from the molten metal to escape through the mold thickness during casting. Expansion of the backing material during baking is undesirable because it changes the dimensions and thus affects the tolerances of the mold. Moreover, because of the interaction between the backing layer and the facing layer, the expansion of the backing layer causes distortions and irregularities in the mold. Unless the backing and facing layers exhibit nearly identical thermal expansion characteristics, those layers will "fight" each other. Moreover, in some instances, the facing layer resists expansion during baking altogether.
Thus, heretofore, for low-tolerance applications, it has oftentimes been necessary to resort to either extensive finishing procedures of the metal castings after they are pulled from the molds, or, manufacturing the metal castings by machining rather than molding. The main and obvious disadvantage of these procedures is that they are labor-intensive and thus, as compared to pure molding procedures, are extremely expensive.
Other problems also are inherent in these prior art two-layer mold fabrication methods. Consider a mold fabricated by first pouring a backing composition about an oversized pattern, permitting the backing layer to gel, and then pouring the facing composition in the gap between the gelled backing layer and the dimensionally-correct pattern. In these circumstances, it is very difficult to obtain a facing layer that is both of sufficient thickness to resist the heat of the molten metal and which adequately adheres to the backing layer. This is because there is no reliable way to judge the optimal gelling time of the facing layer. The optimal gelling time depends on a variety of factors, such as average particle size, volumes of refractory material, gelling agent, accelerator, and water, mixing time, etc., all of which tend to differ with each slurry prepared. If the facing layer is not permitted to gel for a long enough period of time, it will run after the two-layered mold is removed from the pattern. The facing layer will thus develop thinned areas incapable of resisting the heat of the molten metal. As a result, when the molten metal is poured into the mold, the backing layer will melt and create slag in the surface of the casting. This, in turn, will further cause changes in the shape of the mold and thus affect tolerances. In contrast, if the facing layer is permitted to gel for too long a period of time, it will not adhere well to the backing layer. These problems are encountered even when the facing layer is formed first.
Thus, a method for producing ceramic molds that have very low tolerances, as well as consistency of those low tolerances among different molds made from the same pattern, and that avoids these discussed problems of the prior art methods, is desirable.